Unfortunately, command-not-found uses the arch database.
It would be nice if somebody would write some analogous thing for gentoo, but unfortunately for gentoo this is not so simple:
One would have to use e.g. the portagefilelist.de database, but this either means an online access for every typo (which IMHO is not so good, although perhaps in some cases to have this option is nice) or to download the corresponding database locally. I do not know whether the latter is technically simple and juristically possible. In addition some means would have to be provided to update the local database.

But if this would be bound to command-not-found this would mean an online request to database for every typo. This produces a lot of undesired traffic. It can even be a security issue if you use it for "trusted" users which should better not access the internet at all.
In contrast, command-not-found downloads the full database to the local machine which makes more sense for such an approach IMHO.

Yeah, I see that they host snapshots to download so I guess it could be adapted to use it locally.

Yes, but somebody would have to write the code to download it and update and query the local database. I will probably not have the time to do that now. Moreover, I am not sure how this is concerning legal matters and whether they would like it and their server would stand it if there is such a tool which does this automatically, since probably there is a reason why they have written "use with care" above the snapshot...

Yeah, I see that they host snapshots to download so I guess it could be adapted to use it locally.

Yes, but somebody would have to write the code to download it and update and query the local database. I will probably not have the time to do that now. Moreover, I am not sure how this is concerning legal matters and whether they would like it and their server would stand it if there is such a tool which does this automatically, since probably there is a reason why they have written "use with care" above the snapshot...

wget in a cron once a week; legal is okay as they provide it, shouldn't hurt their servers but probably should be checked up with them, otherwise mirroring it could be appropriate; then simply open the database and run a query against it. The code involved is as long as one of our posts; but indeed, how often to download it and having reliable hosting that one is permitted to use for this purpose are more concerning matters.